Answer: b. They both supported the annexation of Texas.
Explanation:
The most significant aspect of James Polk being elected President of the United States at the same time Anson Jones was elected President of Texas was that both Polk and Jones supported the annexation of Texas. Therefore, the correct answer is:
b. They both supported the annexation of Texas.
This shared support for annexation was significant because it aligned the leadership of the United States and Texas in their goals and aspirations regarding Texas becoming part of the United States. This ultimately paved the way for the annexation of Texas by the United States during Polk's presidency in 1845.